Mircoporous hollow fibers are well known in the art as are the advantages which can be derived therefrom. For example, microporous hollow fibers possess a larger surface area per unit volume than a flat film of similar porous configuration when employed in a separatory device. Consequently, they are capable of minimizing priming requirements of devices employing the same which are used for filtration, and a variety of other purposes. A low priming requirement is especially significant in the separation of human blood into plasma and its cellular components commonly referred to as blood plasmapheresis since regulations require that no more than about 500 ml of whole blood can be outside the body at any given time during a plasmapheresis procedure.
Centrifugation techniques are currently utilized for most plasmapheretic applications. However, these techniques are conducted on a batch basis and have several drawbacks related to processing time, cost effectiveness and safety.
Consequently, the use of membrane filtration technology for the continuous flow plasmapheresis as an alternative to existing centrifugal techniques is the subject of an ongoing investigation.
It has been observed that when blood flows in narrow channels, such as a hollow fiber, red blood cells tend to migrate toward the axis of the path, leaving a "cell poor" layer at the periphery. If the wall of the channel is permeable (e.g., a microporous membrane), plasma can be collected without affecting the cells. In such a system, two forces act on the blood cells. The first is a "drag force" that tends to draw the cells to the filtering wall, while the second is a "repulsive force" that moves the cells toward the axis of the channel. If the repulsive force, which is a function of the shear rate of the system, is higher than the drag force, contact of the cells with the filtering surface is minimized and plasma can be rapidly collected.
In order for a microporous hollow fiber to be suitable for use in plasmapheresis applications the pore size, pore density, thickness, and structure of the fiber must be controlled to achieve filtration of only the blood plasma at an acceptable filtration rate. This requires that the hollow fiber have a pore size which is large enough to pass the plasma protein molecules through the fiber wall and yet small enough to prevent passage of the blood cells therethrough. In addition, the mechanical properties of the microporous hollow fiber must be sufficient to prevent rupture of the fiber wall at the transmembrane pressure differential employed during the procedure.
In addition to plasmapheresis procedures, there are a number of other separations which are necessary in certain industrial operations where hollow fiber membranes of the type contemplated by this invention may also be advantageously employed. In all such applications, the process economics are highly dependent upon the transport rate of various components across the fiber wall and the ability of the fiber wall to discriminate various components. That is, an efficient hollow fiber membrane must not only have a high permeability or transport rate but also possess a high degree of selectivity. The difficiencies of the hollow fibers of the prior art center primarily on the inability to achieve an acceptable balance of mechanical properties, selectivity, and permeability.
For example, cellulosic and polycarbonate membranes generally possess poor mechanical properties and rupture relatively easily.
There is a wide variety of techniques known for imparting a porous structure to polymeric fibers or films. For example, U.S. Pat. No. 3,839,516 is directed to a process for preparing a microporous film by a solvent stretching technique which relies on the same general principles as that of the subject invention for developing a microporous structure in a hollow fiber. However, this patent is directed to films and not hollow fibers. Consequently, there is no recognition that the degree of orientation of the precursor hollow fiber which is rendered microporous in the present invention must be controlled to render them processable on a continuous basis and to control and improve the permeability thereof. Improvements in the solvent stretch film process of U.S. Pat. No. 3,839,516 may be found in U.S. Patent Application Ser. No. 44,801 filed June 1, 1979 and U.S. Patent Application Ser. No. 44,805 filed June 1, 1979. These applications also fail to suggest the importance in providing a precursor hollow fiber with a specified degree of orientation.
U.S. Pat. No. 4,055,702 discloses a process wherein interconnecting microvoids are imparted to a solid fiber and the microvoids are impregnated with various additives. This is achieved by cold drawing an undrawn or partially drawn melt spun fiber formed from a polyester, polyamide, propylene, or high density polyethylene in the presence of a non-solvent swelling liquid or vapor drawing medium to achieve a localized reduction in the diameter of the fiber. The temperature of the drawing medium, however, must be below the effective glass transition temperature of the fiber. Moreover, drawing media which swell the undrawn fiber for more than about 2% of the dry volume are not considered as suitable. This patent also fails to recognize the importance of molecular orientation of the precursor fiber.
U.S. Pat. No. 3,325,342 discloses a process wherein a non-crystalline, unoriented as-spun polyamide solid fiber yarn is treated in an aqueous swelling agent to develop crystallinity and then drawn to crystallize the yarn. The drawn yarn exhibits a lower density than the undrawn yarn due to the presence of microscopic voids produced by the swelling agent during the crystallization step. It is not stated whether such microvoids are interconnecting which is essential in the present invention. Moreover, the as-spun fiber which is drawn is unoriented and non-crystalline both of which conditions are unacceptable in the present invention.
None of the above described patents is directed toward preparing an open-celled, microporous, hollow fiber in accordance with the procedures of the present invention.
It is therefore an object of the present invention to provide a process for preparing microporous, open-celled hollow fibers on both a batch, and particularly on a continuous basis which exhibit an improved balance of selectivity, permeability, mechanical strength properties and processability.
It is another object of the present invention to provide microporous, open-celled hollow fibers which have utility as separatory membranes.
It is a further object of the present invention to provide microporous open-celled hollow fibers which have particular utility in plasmapheresis applications.
These and other objects and features of the invention will become apparent from the claims and from the following description.